Digital fabrication of vocal tract models from magnetic resonance imaging during expert pitch bending on the harmonica

Abstract

Expressive pitch bending on the harmonica requires the acoustic coupling of contiguous free reeds, an effect known to arise from highly constrained vocal postures. These techniques have been difficult to demonstrate and instructors often rely on associated vowels to help novices achieve the required constriction of the vocal tract. Magnetic Resonance Imaging (MRI) was recently used to expose the precise vocal contours involved in such expert pitch bends (Egbert, etal., J. Acoust. Soc. Am. 133, 3590 (2013). To further this investigation, we process the MRI data using 3D slicing software, generating digital models of the vocal tract during sustained bends. In addition to providing volumetric data, the models are also fabricated with fusion deposition modeling and tested on real harmonicas. These tests reveal error tolerances in the conversion from MRI slices to 3d printed models when working with geometries that are highly constrained by a desired acoustic output. Furthermore, comparisons between human performance and simulated output provide clues to the contribution of factors not reproduced in the plastic models such as pharynx dilation.